1. Field of the Invention
The present invention relates to an amplifying apparatus suitable for amplification of, among other things, a detection signal output by an object detecting apparatus. In particular, the present invention relates to an amplifying apparatus which has a high direct-current operating-point stability and a large dynamic range as well as an alternating-current signal gain adjustable independently of the high direct-current operating-point stability.
2. Description of the Related Art
As shown in FIG. 24, a conventional amplifying apparatus used for amplification of a detection signal output by an object detecting apparatus comprises an alternating-current coupling inverting amplifier 100, a comparator 103, a direct-current power supply 104, a signal input terminal 105 and a signal output terminal 106. The alternating-current coupling inverting amplifier 100 further comprises amplifying transistors 1001, 100b and 100c which are each capacitor-coupled. The comparator 103 further comprises an operational amplifier 101 and a potentiometer 102.
As a signal from an object detecting sensor, not shown in the figure, is supplied to the signal input terminal 105. The signal is amplified by the alternating-current coupling inverting amplifier 100 employed by the conventional amplifying apparatus to a predetermined level before being supplied to the comparator 103. The operational amplifier 101 employed in the comparator 103 compares the amplified signal supplied thereto to a reference voltage generated by the potentiometer 102. The comparison produces an output signal higher than a predetermined level at the signal output terminal 106.
By the way, the amplifying apparatus of this type employs the amplifying transistors 100a, 100b and 100c which are each capacitor-coupled as described above. In the case of a pulse signal having a small duty cycle or a signal with a duty cycle varying with the lapse of time, the direct-current operating point of the alternating-current coupling inverting amplifier 100 for such a signal fluctuates so that the signal cannot be amplified accurately. In addition, the amplifying apparatus has a problem that a signal amplified by the alternating-current coupling inverting amplifier 100 with a fluctuating direct-current operating point causes the comparator 103 to produce a varying output-signal level due to the fluctuation of the direct-current operating point.
In order to solve the problem, a new technique is introduced. In this technique, the alternating-current coupling inverting amplifier 100 is replaced by an operational amplifier or an amplifying circuit which has a non-differential input and can thus be used for a high-frequency operation. However, the new technique introduces other problems. That is to say, the operational amplifier cannot be used for high-frequency operation or, if it can, it consumes much power, requiring a high direct-current operating voltage. As for the amplifying circuit proposed in the new technique, its dynamic range is small and its direct-current operating point is unstable.
It is an object of the present invention to provide an amplifying apparatus which can solve the technical problems described above.